The invention relates to a projection display device comprising at least one liquid crystal display device having a layer of liquid crystalline material between two supporting plates provided with electrodes which define at least one pixel, via which electrodes a voltage can be applied across the pixel.
A device of this type is used, for example, in projection television and for other video applications.
The invention also relates to a liquid crystal display device for use in such a device.
When using liquid crystal display devices for color projection display, a display panel having, for example, red, green and blue pixels on one panel can be used. The optical system for this device is simpler than for devices in which a separate display device is used for each color and in which the resultant pictures are projected one across the other on a screen. In the latter type it is possible to design each panel optimally from an optical point of view by optimizing for each of the three colors the optical path length d..DELTA.n (d: thickness of liquid crystal layer; .DELTA.n: difference in refractive index between ordinary and extraordinary wave) for the central wavelength associated with this color.
At the central wavelength for red, green and blue .lambda..sub.R, .lambda..sub.G and .lambda..sub.B such thicknesses d.sub.R, d.sub.G, d.sub.B and .DELTA.n values .DELTA.n.sub.R, .DELTA.n.sub.G and .DELTA.n.sub.B are chosen that ##EQU1##
(or the value of another Gooch and Tarry extremum).
This can be effected, for example, by varying the cell thickness for a selected material having a given .DELTA.n or by selecting a different liquid crystal material for each colour for a selected fixed thickness. However, this leads to superfluous storage and production control problems.
EP-A 0,311,116 describes a solution to these problems, with the transmission for each one of the three colours red, green and blue being optimized by adjusting the polarizers for each colour differently with respect to each other, which polarizers are present at both sides of the layer of liquid crystalline material. The values of d and .DELTA.n are then chosen to be such that it holds for green .lambda..sub.G at the central wavelength that: ##EQU2##
For parallel polarizers the transmission is zero at zero voltage. For the blue and red light paths the same values of d and .DELTA.n are used, but the mutual position of the polarizers is shifted.
A first drawback is that the highest value of the transmission for blue and red is not optimum because the polarizers are no longer parallel.
Moreover, it appears that a shifting of the polarizer positions is necessary due to the temperature dependence, notably of .DELTA.n, at temperature variations (particularly an increase of temperature which may be caused by the high beam intensity in projection display). This is effected in the relevant device by rotating the polarizers with respect to each other so that also for the green beam they are no longer parallel, which has a detrimental effect on the transmission.
The influence of the temperature on the transmission curves can be very different due to the different rotations of the polarizers so that for the three different colors the maximum transmission at the highest operating temperature may vary considerably as a percentage of the maximum transmission at room temperature.
Moreover, the mechanical adjustment of the polarizers relative to one another is slow and cumbersome.